The invention relates to a self alignment device for an optical infrared image observation system for controlling a picture taking system, and particularly an optical system, with respect to a given direction. The invention finds particularly interesting applications in airborne picture taking and sighting systems on board mobile missiles.
For infrared image observation (3-5 micrometers, 8-12 micrometers etc.) numerous optical systems have been developed. They are often called FLIR (forward looking infrared) when their resolution allows a television type video image to be obtained. These systems may be broken down into two subassemblies: a focusing optical system and a camera subassembly.
The camera subassembly includes the following elements: a cooled infrared detector block, a scanning system and an internal optical system for analysing the image plane of the focusing optics (intermediate image plane).
These systems are often placed in very severe environments (vibrations, thermal stresses) which may give rise to movements of the optical systems mentioned above (focusing optics, camera subassemblies), with respect to each other or deformations of said systems.
That may cause an appreciable degradation of their functional performance with respect to their values at rest.
One of the most fundamental requirements is optical alignment of such as system with respect to a given direction of space or of the equipment. A name commonly used is the term "sighting line". In addition, in the case of an articulated system, this alignment may have a direction different from that desired because of the optical, mechanical and electrical imperfection of the movements used. In the operational functioning of some of these systems these deformations are critical with respect to the desired performances (very accurate deviation measurement with respect to the target) and may therefore impair the operation of the system.
The aim of the present invention is to be able to measure the amplitude of these deformations so as then to take them into account for correcting and controlling the sighting line position of these systems.
In the technique, systems are known for measuring these deformations. These systems use an auxiliary light source emitting a light beam through the optical system (focusing optical system) whose alignment it is desired to adjust with respect to the sighting line and an auxiliary detector for detecting the light beam emitted by the auxiliary light source. Semi-reflecting mirrors are further required for separating the light beam emitted by the light source from the beam coming from the object observed and received by the focusing optical system. These systems have the drawback of reducing the photometric performances of the system and of reducing the spectral band which can be used because of the presence of the semi-reflecting mirrors and the auxiliary light sources. In addition, the auxiliary light sources may be at the origin of light or parasite images disturbing the operation of these systems.
This is why the invention relates to a system for avoiding these drawbacks.
The system is based on the use of an effect observed with thermal detectors which are generally cooled. This so-called "narcissus" effect occurs when, during scanning of the image field, the detector sees its own image (or that of its cold diaphragm) obtained by self collimation on an optical diopter, cooled image on a warm background. This effect is reduced as much as possible during calculation of the optical combination of the focusing lens essentially by suppressing any surface close to self collimation for the sensitive surface.
The system of the invention has the particularity of using this "narcissus" effect which is usually harmful in scanned thermal optical systems.